The invention relates to a method for structuring and operating a cryo probe head for the transmission and/or reception of radio-frequency signals for nuclear magnetic resonance measurements, with at least one heat exchanger for cooling one or more heat sources, in particular, components of the cryo probe head, wherein a cryogenic fluid is supplied to the heat exchanger and the heat exchanger has at least one contact element that ensures a connection with good thermal conduction between the cryogenic fluid and the heat source.
Such a configuration is known from DE 103 40 352 A1.
In a configuration for measurement by means of nuclear magnetic resonance, a probe head is placed in the strong steady-state magnetic field of a typically superconducting magnet. The sample to be measured is introduced into this. The probe head contains radio-frequency coils and resonators that are used to excite nuclear spins and to receive the signals generated by the excited nuclear spins. Because the nuclear spin signals are generally very weak, there are easily prone to interference. For that reason, the signal-to-noise (S/N) ratio is a very important performance criterion in nuclear magnetic resonance. One common procedure for improving the S/N ratio is cooling the relevant components to the lowest possible temperature. This is done using cryo probe heads.
To cool cryo probe heads to cryogenic temperatures, cryocoolers with closed gas circuits are generally used, preferably with helium as the fluid and based on Gifford-McMahon, Stirling, or Joule-Thomson processes.
A further cooling method is evaporation of cryogenic fluids within the probe head. Liquid is run through tubes into the probe head, where the thermal load causes it to evaporate.
One feature of any cooling mechanism is a fluid that provides the necessary cooling power for heat dissipation and a thermal contact system that permits the transfer of heat from the component to be cooled to the heat transfer medium=fluid. Such a device is generally termed a heat exchanger.
Irrespective of the characteristics of a cooling system and its auxiliary equipment, an NMR detection system must be operated with as little disturbance as possible and with low operating and maintenance costs.
This invention described below is for cooling components of an NMR detection system with the aim of providing cooling that is as simple and efficient as possible with minimum effort and minimum susceptibility to internal or external disturbances.
Operation of cryocoolers requires a number of items of auxiliary equipment such as compressors, heat sinks, pumps, etc., which makes installation and operation correspondingly costly both in terms of maintenance effort and operating costs. Moreover, the use of rotating or linearly moving components in the equipment leads to transmission of mechanical vibrations into the probe head. Mechanical vibrations that are transmitted to the probe head can have a considerable adverse influence on the NMR signal.
According to the usual prior art, a heat exchanger is supplied through a flow inlet with gaseous or liquid cooling fluid, which exits the heat exchanger through a flow outlet. By means of a contact element with good thermal conduction, a heat exchanger is connected to a heat source, wherein the latter is kept at a desired temperature by dissipating a certain heat load. The heat source can, for example, be an RF resonator or a signal amplifier.
In available systems, the connections between the flow inlet and the flow outlet are constituted as cooling ducts that enable a fluid to flow through and heat to be transferred from a heat source to a cooling fluid in the thermal contact system through structures that may be routed in any way. The cooling conduits can extend helically around a thermal contact system or be spiral and embedded in the thermal contact system. Other embodiments of the cooling ducts are also possible.
Both cooling by evaporation of a fluid cryogen and cooling with cryogenic gases in cryocoolers are used in standard commercial NMR detection systems, although considerable disadvantages are encountered in systems according to the state of the art.
Cooling of cryo probe heads using thermodynamic circulatory processes implemented in cryocoolers is mentioned in DE 103 403 52 A1 and U.S. Pat. No. 5,508,613 A.
Cooling by evaporation of liquid helium or nitrogen within a cryo probe head is described in EP 0 782 005 A1.
Problems with the two-phase flow of a cryogenic fluid in a tube, the associated unstable states, and their effect on thermal transfer properties was discussed by Qi et al. in the International Journal of Heat and Mass Transfer (Vol. 50, Issue 25/26, Page 4999-5016).
An NMR RF coil cooled with cryogenic liquid was described by Styles at al. in the Journal of Magnetic Resonance (Vol. 60, pages 397-404, 1984). In the configuration described therein, the RF coil is constituted as a tube through which cryogenic fluid flows. Within the coil, evaporation of the fluid and thus cooling of the RF coil is caused by the thermal power of the RF coil. However, such a configuration has considerable disadvantages. The formal design of the coil would be heavily restricted if it had to be constituted as tubes. During evaporation and the associated rapid density change, the flow of the gas phase is greatly accelerated. Because the design does not provide for phase separation, the density change affects the liquid phase and the entire fluid transportation in the form of thermoacoustic vibrations. The cryogenic fluid compressed through the coil under pressure causes vibrations, possibly resulting in susceptibility changes in the coil and mechanical vibrations.
DE 40 13 111 A1 describes a system in which the coil is not in direct contact with the cryogenic fluid but is connected to it through a heat-conducting connection. However, the problem of vibrations persists, despite physical separation of the functions “cooling” and “RF reception”, since cryogenic fluid still has to be transported to the heat-conducting connection and the problem of vibrations in the fluid conduits remains.
The use of liquid helium for coil cooling is explicitly stated in JP 2008 241 493 A and WO 03/023433 A1. In both sources, liquid helium is introduced into a heat exchanger through a tube from below. Disturbances in the fluid transportation are thus propagated, undamped into the heat exchanger. Because of the flow inlet from below, gas bubbles occurring in the inflow conduit must flow through the entire liquid bath on entering the heat exchanger, resulting in very unsettled bubbling of the liquid level and causing considerable mechanical vibration. A further weakness is the insufficiently settable or non-settable level of the liquid cryogen and the fluctuating cooling power.
The necessity of thermal decoupling of multiple RF coils is discussed in DE 103 40 352 A1. In the case of the device described there, a separate heat exchanger, through which gaseous fluid flows, is used for each coil, which is incompatible with a compact design. Moreover, in the proposed serial connection gas cooling for NMR detectors the inflow into one heat exchanger is always warmer than the inflow in the preceding heat exchanger. To achieve a temperature at the same level, active closed-loop control has to be used.